The present invention relates to improvements in angle sensors.
In U.S. Pat. No. 5,794,355 a container is formed by a pair of concentrically aligned hemispherical surfaces. The container is filled with a viscous fluid and a bubble of a lighter-weight fluid and placed between a radiation source and a radiation detector. The bubble changes position within the container when the sensor is moved, transmitting a beam of radiation from the radiation source through the bubble to activate a section of the radiation detector while the remainder of the radiation is blocked by the fluid. Two-axis angle sensing is achieved by the use of a radiation detector comprising a two-dimensional array of grid elements (eg photodiodes).
The arrangement of U.S. Pat. No. 5,794,355 suffers a number of problems. Firstly, in order to achieve a measurement range of n measurement units the two-dimensional radiation detector must have n*n grid elements. As n increases the radiation detector can become large and expensive. Secondly, random measurement errors in the signal from the radiation detector can be large.
An alternative method of achieving two-axis angle sensing is suggested in U.S. Pat. No. 5,218,771 at column 4 lines 15-19. This suggests using two angular motion detectors, mounted with their respective central axes oriented perpendicular to each other. However the arrangement of U.S. Pat. No. 5,218,771 still suffers from the problem of random measurement error.
In accordance with a first aspect of the present invention there is provided a two-axis angle sensor comprising
a first bubble chamber containing two fluids of different characteristics such that a bubble is formed in the chamber;
a first bubble detector for generating a signal indicative of the orientation of the first bubble chamber with respect to a first detector axis by sensing the position of the bubble in the first bubble chamber;
a second bubble chamber containing two fluids of different characteristics such that a bubble is formed in the chamber;
a second bubble detector for generating a signal indicative of the orientation of the second bubble chamber with respect to a second detector axis by sensing the position of the bubble in the second bubble chamber, and
a processor for calculating the angle of the sensor with respect to first and second measurement axes by combining the signals from the first and second bubble detectors in accordance with a predetermined algorithm, wherein the measurement axes are angularly offset from the detector axes.
Instead of aligning the bubble detectors and bubble chambers with the measurement axes and taking direct independent readings from the bubble detectors, we offset the bubble detectors and bubble chambers from the measurement axes and combine the signals from the bubble detectors. We have recognised that if the detector signals suffer from random errors with a gaussian distribution then the error associated with one bubble detector will tend to cancel out the error associated with the other bubble detector. Thus the combined measurement will be more accurate than a single independent measurement.
The bubble chambers and bubble detectors can then be provided in a sensor housing which is aligned with the measurement axes. Thus for example the sensor housing may comprise a joystick which is shaped to be gripped by the hand such that the xe2x80x9cforward/reverse pitchxe2x80x9d direction is aligned with one measurement axis and the xe2x80x9cleft/right rollxe2x80x9d direction is aligned with the other measurement axis. Alternatively the sensor may be a xe2x80x9cmousexe2x80x9d type computer input device which is gripped by the hand but not constrained to be used on a surface. In this case the sensor housing will be shaped to be gripped by one or more hands such that the xe2x80x9cforward/reversexe2x80x9d direction is aligned with one measurement axis and the xe2x80x9cleft/right rollxe2x80x9d direction is aligned with the other measurement axis. One or more buttons may also be provided in a position to ensure that the sensor housing is gripped in the preferred orientation. In a further alternative the sensor may be mounted in a vehicle or aeroplane with one of the measurement axes aligned with the direction of forward movement.
A variety of algorithms may be used, depending on the outputs of the detectors. In a preferred example the predetermined algorithm comprises:
summing a pair of values derived from the bubble detector signals to calculate the angle of the sensor with respect to the first measurement axis; and
subtracting a pair of values derived from the bubble detector signals to calculate. the angle of the sensor with respect to the second measurement axis.
The precise form of the algorithm will depend on the angular relationship of the axes. These axes may be offset from each other by any desired angle. However in a preferred example the detector axes are arranged substantially at right angles to each other, and the measurement axes are arranged substantially at 45 degrees to the detector axes. This enables the bubble detector signals to be simply added or subtracted without requiring either signal to be scaled up or down with respect to the other signal before addition or subtraction.
A further problem with the arrangement of U.S. Pat. No. 5,794,355 is that some radiation may be transmitted through the viscous fluid making it difficult to detect the bubble. U.S. Pat. No. 5,794,355 addresses this problem by adding a dye to the viscous fluid to absorb the radiation. In addition there is no design freedom in the positioning of the radiation detectorxe2x80x94ie. it must be positioned directly behind the container on the opposite side to the radiation source.
In U.S. Pat. No. 5,218,771 the materials forming the bubble and the liquid are chosen so that the interface surface between them is highly reflective. Thus the detector detects light reflected from the surface of the bubble. A problem with this arrangement is that there is no design freedom in the positioning of the radiation detectorxe2x80x94ie. it must be positioned to receive the reflected light from the bubble.
In accordance with a second aspect of the present invention there is provided an angle sensor comprising
a bubble chamber containing two fluids of different characteristics such that a bubble is formed in the chamber;
a radiation source for illuminating the bubble with radiation whereby the radiation is refracted by the bubble; and
a radiation detector positioned to detect the refracted radiation from the bubble and generate a signal indicative of the angle of the bubble chamber.
In contrast to the conventional approach we detect refracted radiation from the bubble. As a result it is not necessary to dye the fluid. Furthermore we can accommodate different arrangements for the radiation source and radiation detector by selecting appropriate refractive indices for the bubble chamber and the two fluids. This is not possible in prior art systems which detect reflected or transmitted light since the angle of reflection or transmission is fixed regardless of the refractive indices.
In a preferred arrangement an interface is positioned to receive the refracted radiation from the bubble and deliver the refracted radiation to the radiation detector. This increases the amount of refracted radiation falling on the detector, thus improving the measurement accuracy.
A number of different interfaces may be provided. For instance the interface may comprise a light guide such as a fibre-optic cable. In one arrangement the interface comprises one or more lenses positioned between the bubble chamber and the radiation detector for focusing the refracted radiation onto the radiation detector. Alternatively the interface may comprise a radiation transmissive projection in the bubble chamber. The projection typically has a face arranged at an angle such that refracted radiation from the bubble passes through the face, and radiation from other directions is internally reflected by the face back into the bubble chamber.
In order to absorb radiation which has not been refracted by the bubble, the bubble chamber preferably has a radiation absorbent portion and a radiation transmissive window positioned adjacent to the bubble. This may be achieved by manufacturing the bubble chamber from two different materials having different radiation absorption characteristics. However in a preferred arrangement the radiation absorbent portion is formed by a coating of radiation absorbent material.
In the arrangement of U.S. Pat. No. 5,218,771 a single light source and a pair of light sensors are placed in a T-shaped arrangement with the sensors on opposite sides of the bubble chamber. Thus each sensor detects light which has been reflected to the right or to the left by the bubble. The position of the bubble is deduced by analysing the outputs of the sensors. A problem with this system is that a complex calculation must be performed using a standard table lookup scheme to deduce the position of the bubble. Therefore in a preferred embodiment of the present invention the radiation detector comprises a position sensitive detector, the position of the refracted radiation on the detector being indicative of the position of the bubble in the bubble chamber. This provides a simpler method of position measurement than U.S. Pat. No. 5,218,771.
Typically the position sensitive detector comprises an array of detectors, such as a charge-coupled device (CCD). Typically the array of detectors are arranged in a substantially straight line. If the bubble moves along a curved path then preferably a cylinder lens is provided to project the curved path onto a straight line detector.
In order to ensure that refracted radiation from the bubble falls onto the detector it is important to arrange the radiation source at the correct position. In a preferred embodiment the radiation source is positioned to illuminate the bubble with a beam of radiation which is offset from the centre of the bubble. This arrangement ensures that the refracted output beam subtends the illuminating beam at an angle. Conveniently a radiation guide is provided for guiding radiation between the radiation source and the bubble at the desired incident angle.
Typically the refractive index of the fluid forming the bubble is lower than the refractive index of the other fluid. In this arrangement the bubble causes the illuminating radiation to diverge, increasing the need for one or more lenses for focusing the refracted radiation onto the radiation detector. Conveniently bubble comprises a gas bubble (although two liquids could also be used).
Conveniently the radiation source comprises an infrared radiation source. This enables the radiation detector to be insensitive to stray visible background optical radiation.
Where two or more angle sensors are used, as for example in accordance with the first aspect of the invention, each may have its own radiation source and detector or they may use common sources and/or detectors by, for example, operating at different frequencies depending upon the sensor concerned.
An inertial sensor known as a vibrating structure gyroscope (VSG) is described in EP-A-0457541, JP-A-09050343 and xe2x80x9cPS/2: GYROSCOPIC MOUSE DEVICExe2x80x9d, IBM TECHNICAL DISCLOSURE BULLETIN, vol. 34, no. 11, April 1, 1992, pages 89-90. A common problem with VSGs is the need to manually set the device or provide a fixed or controlled horizon.
In accordance with a third aspect of the present invention there is provided an angle sensor comprising
a vibrating structure gyroscope (VSG) for generating a VSG signal indicative of the orientation of the sensor;
a bubble chamber containing two fluids of different characteristics such that a bubble is formed in the chamber;
a bubble detector for generating a calibration signal responsive to the position of the bubble in the bubble chamber; and
a processor for calibrating the VSG signal from the calibration signal in accordance with a predetermined calibration algorithm.
This provides a robust and cost effective method of calibrating the VSG signal.
Any calibration algorithm may be used but in a preferred arrangement the bubble detector generates the calibration signal when the bubble passes a centre point. The calibration signal thus provides an artificial horizon to calibrate the VSG signal.
A further problem with conventional angle sensors is that the sensor does not provide any resistance to operator input.
In accordance with a fourth aspect of the present invention there is provided an angle sensor comprising
means for generating a signal indicative of the orientation of the sensor; and
a flywheel which provides a gyroscopic force to oppose tilting of the sensor.
The flywheel creates the effect of a stable platform which increases operator feedback and dampens over-enthusiastic input from the operator. In a preferred arrangement means are provided to vary the speed of rotation of the flywheel in order to vary the gyroscopic force, for instance in conjunction with a game being played using the angle sensor as an input device.
The means for generating a signal indicative of the orientation of the sensor may comprise a VSG. However in a preferred arrangement the means for generating a signal indicative of the orientation of the sensor comprises:
a bubble chamber containing two fluids of different characteristics such that a bubble is formed in the chamber; and
a bubble detector for generating a signal indicative of the orientation of the first bubble chamber by sensing the position of the bubble in the bubble chamber.
Typically the flywheel is driven by a motor, for example a DC or three phase AC motor.
A further problem associated with bubble angle sensors is that it can be difficult to ensure that the bubble is of the correct size. If the bubble is too large or too small the accuracy of the sensor may be compromised.
In accordance with a fifth aspect of the present invention there is provided a method of manufacturing an angle sensor, the method comprising:
providing a container with an open end;
substantially filling the container with liquid;
providing a lid with one or more internal recesses;
closing the liquid filled container with the lid to form an enclosed bubble chamber, wherein on closing the container a gas bubble becomes trapped in the or each internal recess; and
arranging a bubble detector to generate a signal indicative of the orientation of the bubble chamber by sensing the position of the gas bubble in the bubble chamber.
A number of recesses may be provided but in a preferred arrangement only a single recess is provided, the recess having a volume corresponding with the volume of bubble required. Typically the lid is pivotally attached to the container, and rotated to close the container.
In all these aspects of the invention, the bubble chamber preferably has a D-shape but it could also be circular or take some other convenient form.
The angle sensor according to the first, second, third, fourth and fifth aspects of the present invention may be employed in a variety of applications. However in a preferred example the angle sensor is incorporated in a user input device, for instance for a computer game console or a PC. The input device may comprise a joystick which is controlled by one hand, or a xe2x80x9cmousexe2x80x9d type input device which is held in both hands and rotated in free space. Alternatively the angle sensor may be used to detect the orientation of a vehicle or aeroplane.